GB1602920A

GB1602920A - Dc-dc converters

Info

Publication number: GB1602920A
Application number: GB2518178A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1977-10-31
Filing date: 1978-05-31
Publication date: 1981-11-18
Also published as: DE2748757C2; DE2748757A1

Description

(54) IMPROVEMENTS IN OR RELATING TO D.C.-D.C.

CONVERTERS (71) We, SIEMENS AKTIENGESELLS CHAFT, a German Company, of Berlin and Munich, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to D.C.-D.C converters of the type comprising at least one periodically operating switching transistor having a transformer primary winding in its collector circuit, the transformer having a secondary winding providing at least one stabilised rectified D.C. voltage control being effected by means of an outer regulating circuit supplying the switching transistor base current to effect output D.C. voltage stabilisation, together with an inner regulating circuit which operates each time the switching transistor is rendered conductive to determine the disconnection time thereof, means being provided to use the collectoremitter voltage of the switching transistor at any instant as a controlling value, on the overshooting of a given theoretical value being sensed, said means causing the flow of base current to the switching transistor to be interrupted, the theoretical value corresponds to a voltage at which the transistor is no longer over-saturated.

One D.C.-D.C. converter of this kind is described in our U.K. Patent Application No. 27057/75 (Specification No. 1,528,358), in which a load-dependent base current for the switching transistor is produced by the outer regulating circuit and switching on and off by the inner regulating circuit at the most favourable switching times. In order to ensure short switch-off times across the switching transistor and thus reduce power losses, this transistor cannot be allowed to be switched out of saturation, and for this purpose the residual voltage across the switching transistor is measured via the inner regulating circuit. This known converter operates satisfactorily when it is operated from a rectified mains supply voltage. However, problems arise when a low input voltage, for example a D.C. battery voltage of only 24 V, is employed. In this case the switching transistor must be supplied with a very high base current in order to be able to switch the same level of power. If it were desired to produce this high base current with the normal voltage regulator from the known circuit, an unviably high power loss would result.

One object of the present invention is to provide an improved circuit for a D.C.-D.C. converter of the type described in the introduction, which can be operated in such manner that, even with a low input voltage the switching transistor is driven in optimal fashion with a low power loss and maintains these properties within a wide input voltage range, whilst retaining accurate regulation.

The invention consists in a D.C.-D.C.

converter in which at least one periodically operating switching transistor is connected in series with a transformer primary winding that lies in the collector circuit of the switching transistor, and from a secondary winding of which at least one stabilised d.c.

voltage is obtained by rectification, an outer regulating circuit being provided to control the base current to the switching transistor and effect D.C. output voltage stabilisation, and an inner regulating circuit being provided which acts each time the switching transistor has been rendered conductive to determine the disconnection time thereof, the collector-emitter voltage of the switching transistor being used as a controlling value, such that when it overshoots a given theoretical value disconnection of the base current to the switching transistor is effected, said theoetical value corresponding to a voltage at which the transistor is no longer oversatu rated, wherein the base of the switching transistor is preceded by a control switching amplifier stage consisting of a control amplifier transistor and an auxiliary control transformer, the primary winding of the auxiliary transformer being in series with the collectoremitter path of the control amplifier transistor which is connected to the output of the outer regulating circuit, and the output signal from the inner regulating circuit being connected to the base of the control amplifier transistor, whilst a secondary winding of the auxiliary transformer is connected in the base circuit of the switching transistor (V15).

Thus the switching amplifier provided by the invention operates with inductive coupling, and the magnetisation current of the auxiliary control transformer assumes the base control operation for the switching on of the switching transistor. Advantageously, those ends of the two windings of the auxiliary transformer that are free of any high frequency potential are each poled in like fashion and the auxiliary transformer possesses an airgap which is such that it is able to intermediately store the energy for the base operation of the switching transistor.

Expediently, the inductance of the auxiliary transformer is selected to be sufficiently low that during each switching period of the control transistor a significant change in the current flowing through the secondary winding occurs automatically.

The outer regulating circuit (voltage regulator) determines an operating voltage for the switching amplifier. Energy is input into the primary end of the auxiliary transformer for such time as the control transistor is conductive. If the control transistor is blocked by the inner regulating circuit, the auxiliary transformer emits its energy to the base of the switching transistor. The magnitude of the base current is determined by the outer regulating circuit and is initially greater than would be required for the collector current rising linearly from zero. Thus initially the transistor is oversaturated. At the end of the current flow time, the current amplification which is no longer sufficient results in a desaturation of the load switch which is indicated by a rapid rise in its residual voltage. This criterion is established in known manner by the inner regulating cir- cuit which immediately initiates a disconnection by switching on the base current for the amplifier transistor. The base of the load switch is discharged at a high current so that the switch-off time and the switch-off losses can be kept extremely low.

Expediently the auxiliary transformer primary winding, which is arranged in the collector circuit of the control transistor is connected in series with the collector-emitter path of a further transistor which is controlled by the outer regulating circuit and connects a load-dependent D.C. voltage to the switching amplifier. It is also advantageous for the collector circuit of the control transistor to contain a diode connected, for example, between the auxiliary transformer winding and the further transistor. which operates upon blocking of the control transistor to prevent a return to conduction being caused by the energy stored in the auxiliary transformer.

In order to protect the control transistor from excess voltages a series RC-element may be arranged in parallel with the primary winding of the auxiliary transformer.

In order to facilitate a particularly short switch-off time of the switching transistor with as high as possible a discharge current, advantageously there is a capacitor arranged in parallel with the control switching amplifier stage. This capacitor serves as intermediate store and provides for a high current peak when the amplifier transistor is switch on.

Furthermore, the secondary winding of the auxiliary transformer is preferably damped by a parallel-connected resistor.

Preferably, a pulse generator is provided which operates to allow the converter to build up, by supplying a monitoring value input of the inner regulating circuit with short pulses of low voltage. This simulates a low collector voltage across the load switching transistor, so that the switching transistor is temporarily switched conductive via the control switching amplifier stage. Following transition into the normal switching operation, this pulse generator is preferably blocked by switching means, such as a parallel-connected diode. In the case of excess voltages at the input, the oscillations within the converter are broken down as the voltage across the switching transistor no longer pass through zero. In order to prevent the pulse generator attempting to switch-on the switching transistor under these circumstances, at the output of the pulse generator there is expediently provided a voltage divider which is dimensioned in such manner that in the event of an excess voltage, its tapping point, which is connected to the monitoring input of the inner regulating circuit does not undershoot the switching threshold of the inner regulating circuit in respect of the pulses of the pulse generator.

The invention will now be described with reference to the drawings in which: Figure 1 is a block schematic circuit diagram of one exemplary embodiment of a converter constructed in accordance with the invention; and Figure 2 illustrates a circuit arrangement for the converter shown in Figure 1.

The block schematic circuit diagram shown in Figure 1 illustrates the fundamental construction of an exemplary D.C.-D.C.

converter constructed in accordance with the invention, which operates in accordance with the blocking converter principle and as a result of its construction is particularly suitable to produce a high output voltage (e.g.

320 V) with a relatively high output power from a relatively low input voltage. The input voltage is fed to the converter via a radio interference suppression filter F that operates in accordance with the resonance principle, and a parallel capacitor PC is provided for the load switch LS, to form a series oscillating circuit with a transformer UE.

The overall control arrangement ST comprises an inner regulating circuit formed by a residual voltage regulator RR for operating the load switch LS, together with a pulse generator TG which, following the connection of the input voltage, emits short pulses to the load switch and excites the resonant circuit to oscillate. The arrangement ST further comprises an outer regulating circuit formed by a voltage regulator SR that determines the base current. In order to reduce the losses when operating with a low input voltage and a high power, a control switching amplifier stage SV is arranged after the voltage regulator SR and the residual voltage regulator RR of the overall control arrangement ST and before the load switch LS. This control switching amplifier stage SV operates with inductive coupling, and the magnetisation current of the drive transformer assumes the base operation of the load switch LS.

Once the converter has started, the residual voltage regulator RR connects the base current supply to the load switch LS whenever its collector voltage becomes zero. The magnitude of the base current is determined by the voltage regulator SR, and is initially greater than necessary for the collector current, which rises linearly from zero. The transistor which is used as load switch LS is thus initially oversaturated. At the end of the current flow time, the current amplification is no longer sufficient. so that there is a resulting desaturation of the load switch, which is characterised by a rapid increase in its residual voltage. This criterion is recognised by the residual voltage regulator RR which immediately initiates a disconnection, and the base of the load switch LS is discharged with a high negative current. The occurring disconnection times and disconnection losses are exteremely small, as the capacitor PC arranged in parallel to the load switch LS additionally damps the voltage rise.

The output circuit fundamentally comprises a rectifier GR and a storage capacitor SC. In the blocking phase, the transformer UE emits its energy to this output circuit, and the rectifier current falls linearly to zero before the rectifier GR blocks, the voltage across the transformer UE swings back and the load switch LS is reconnected. The storage capacitor SC serves as intermediate store which, in the event of any drop in voltage at the input emits its energy to the output and temporarily maintains the operation of the connected device.

Figure 2 illustrates in detail the circuit for the converter. The input filter F which serves to suppress radio interference is of symmetrical construction, and comprises capacitors C21, C22, C24 and C25, and coils L21 and L22. The filter also comprises a mains fuse F1 and a parallel diode V21 which triggers the fuse in the event of the application of a reversed polarity input voltage.

The load supply portion of the arrangement contains an input shunt capacitor C8, a switching transistor V15 and a transformer T2. A primary winding Liy of the transformer T2 forms a series oscillatory circuit with a capacitor C7. During operation the primary winding Liy is connected via a switching transistor V15 between the operating voltage terminals until sufficient energy is stored in the transformer T2 to maintain the output voltage, for example at 320 V, via the secondary winding L12 and a rectifier diode V 11. The switching transistor V15 is arranged in parallel with a diode V12 which protects the switching transistor from reversal of polarity. The shunt capacitor C10 serves as an intermediate store in the event of breaks in the operating voltage, and serves to obliterate the pulse loading of a subsequently connected device (de-compromisation). Radio interference suppression chokes L1 and L2 isolate any following circuit as far as HF voltages are concerned. Finally the transformer T2 also contains an tertiary winding L13 which produes an auxiliary voltage for the overall control arrangement ST.

As can be seen from Figure 1, the control arrangement ST contains the voltage regulator SR, the residual voltage regulator RR and the switching amplifier SV. The voltage regulator SR fundamentally consists of an operational amplifier D1, which compares the output voltage connected via a voltage divider formed by resistors R4 and R1 with a reference voltage which is established by a Zener diode V2. A resistors R5 and capacitor C3 provide the regulator with a dynamically soft curve in order to prevent any pulse loads occurring at the output from being transferred to the input. A capacitor C1 ensures that the remaining regulation deviation statically becomes zero.

The output of the voltage regulator SR controls the magnitude of the base current to be fed to the switching transistor V 15. For this purpose the operational amplifier D1 emits a load-dependent operating voltage via a transistor V13 and a resistor R15 to the switching amplifier SV which fundamentally contains an auxiliary control transformer Tl and a control transistor V 14. Energy is fed into the primary winding of this transformer TI via diodes V7 and V8 when the transistor V14 is conductive. When the transistor V14 blocks, the transformer Tl emits its energy via a secondary winding L15 to the base of the switching transistor V15. The base current reduces at the end of the current flow time in V15, but this is harmless. and even desirable. For switch-on. the transistor V14 also conducts the discharge current. transformed in the auxiliary control transformer Tl. for the switching transistor V15. In order to achieve as steep as possible a current peak when V14 is switched on. a capacitor C4 is connected in parallel with the control stage to act as an intermediate store. The transformer TI is damped by a resistor R19 which is arranged in parallel with the secondary winding L 15. A series RC-element, composed of a capacitor C5 and a resistor R 17. is arranged in parallel with the primary winding L 14. in order to protect the transistor V 14 from any excess voltage.

The control transistor V14 is driven by the residual voltage regulator RR. A fundamental component of the residual voltage regulator consists of the operational amplifier D3 which interrogates the voltage across the collector of the switching transistors V15 via a resistor R10 and compares this voltage with a theoretical reference voltage. This theoretical reference voltage is formed via a voltage divider comprising resistors TRI 1 and R12 in parallel with a Zener diode V2. As soon as the collector-emitter voltage of the switching transistor V15 overshoots the set value, for example 2 V, the control transistor V14 is switched on and the switching transistor V 15 is blocked.

The build-up of the converter is achieved with the aid of the pulse generator TG.

which has an operational amplifier D2, resistors R3, R6. R7 and R8, a diode V3 and a capacitor C2. When the input voltage has been switched on. firstly a potential 'H" appears at the output of the operational amplifier D2: as a result the capacitor C2 is charged via the resistor R7. As soon as the voltage across the capacitor C2 overshoots the reference value at the input of the operational amplifier D2, the potential "L" appears at the output. so that the capacitor C2 is discharged again via the resistor R8 and the diode V3. In this way the pulse generator emits short pulses via the resistor R9 to the residual voltage regulator RR and thus simulates a low collector voltage in the switching transistor. Thus the switching transistor is switched conductive for two to three micro-seconds via the control amplifier stage and the oscillatory circuit comprising L 11 and C7 commences to oscillate. Following transition into normal switching operation. the pulse generator is blocked by the diode V4.

At operating voltages exceeding a predetermined level, the oscillations in the converter are broken down as the voltage across the switching transistor then no longer passes through zero. In order to prevent the pulse generator from making connection attempts under these circumstances, a voltage divider formed by resistors RIO and R9 is designed in such manner that the pulses from the pulse generator no longer undershoot the switching threshold at the input of the residual voltage regulator. As a result. the switching transistor can no longer be switched on by the pulses of the pulse generator until the operating voltage has fallen to a sufficiently low value. On starting. the overall control arrangement is fed via the diode V9 and via the resistors R14 and R18. When the output voltage has built up. the control arrangement is fed from the tertiary winding L13 via the diode V10 and via the diode V6 and the resistor R13, the requisite auxiliary voltage being set up across the capacitor C6. Here the Zener diode V1 protects the operational amplifiers Dl, D2 and D3 from any excess voltage.

WHAT WE CLAIM IS: 1. A D.C.-D.C. converter in which at least one periodically operating switching transistor is connected in series with a transformer primary winding that lies in the collector circuit of the switching transistor, and from a secondary winding of which at least one stabilised d.c. voltage is obtained by rectification, an outer regulating circuit being provided to control the base current to the switching transistor and effect D.C. output voltage stabilisation, and an inner regulating circuit being provided which acts each time the switching transistor has been rendered conductive to determine the disconnection time thereof, the collector-emitter voltage of the switching transistor being used as a controlling value, such that when it overshoots a given theoretical value disconnection of the base current to the switching transistor is effected, said theoretical value corresponding to a voltage at which the transistor is no longer overaturated. wherein the base of the switching transistor is preceded by a control switching amplifier stage consisting of a control amplifier transistor and an auxiliary control transformer. the primary winding of the auxiliary transformer being in series with the collector-emitter path of the control amplifier transistor. which is connected to the output of the outer regulating circuit. and the output signal from the inner regulating circuit being connected to the base of the control amplifier transistor, whilst a secondary winding of the auxiliary transformer is connected in the base circuit of the switching transistor.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. contains an auxiliary control transformer Tl and a control transistor V 14. Energy is fed into the primary winding of this transformer TI via diodes V7 and V8 when the transistor V14 is conductive. When the transistor V14 blocks, the transformer Tl emits its energy via a secondary winding L15 to the base of the switching transistor V15. The base current reduces at the end of the current flow time in V15, but this is harmless. and even desirable. For switch-on. the transistor V14 also conducts the discharge current. transformed in the auxiliary control transformer Tl. for the switching transistor V15. In order to achieve as steep as possible a current peak when V14 is switched on. a capacitor C4 is connected in parallel with the control stage to act as an intermediate store. The transformer TI is damped by a resistor R19 which is arranged in parallel with the secondary winding L 15. A series RC-element, composed of a capacitor C5 and a resistor R 17. is arranged in parallel with the primary winding L 14. in order to protect the transistor V 14 from any excess voltage. The control transistor V14 is driven by the residual voltage regulator RR. A fundamental component of the residual voltage regulator consists of the operational amplifier D3 which interrogates the voltage across the collector of the switching transistors V15 via a resistor R10 and compares this voltage with a theoretical reference voltage. This theoretical reference voltage is formed via a voltage divider comprising resistors TRI 1 and R12 in parallel with a Zener diode V2. As soon as the collector-emitter voltage of the switching transistor V15 overshoots the set value, for example 2 V, the control transistor V14 is switched on and the switching transistor V 15 is blocked. The build-up of the converter is achieved with the aid of the pulse generator TG. which has an operational amplifier D2, resistors R3, R6. R7 and R8, a diode V3 and a capacitor C2. When the input voltage has been switched on. firstly a potential 'H" appears at the output of the operational amplifier D2: as a result the capacitor C2 is charged via the resistor R7. As soon as the voltage across the capacitor C2 overshoots the reference value at the input of the operational amplifier D2, the potential "L" appears at the output. so that the capacitor C2 is discharged again via the resistor R8 and the diode V3. In this way the pulse generator emits short pulses via the resistor R9 to the residual voltage regulator RR and thus simulates a low collector voltage in the switching transistor. Thus the switching transistor is switched conductive for two to three micro-seconds via the control amplifier stage and the oscillatory circuit comprising L 11 and C7 commences to oscillate. Following transition into normal switching operation. the pulse generator is blocked by the diode V4. At operating voltages exceeding a predetermined level, the oscillations in the converter are broken down as the voltage across the switching transistor then no longer passes through zero. In order to prevent the pulse generator from making connection attempts under these circumstances, a voltage divider formed by resistors RIO and R9 is designed in such manner that the pulses from the pulse generator no longer undershoot the switching threshold at the input of the residual voltage regulator. As a result. the switching transistor can no longer be switched on by the pulses of the pulse generator until the operating voltage has fallen to a sufficiently low value. On starting. the overall control arrangement is fed via the diode V9 and via the resistors R14 and R18. When the output voltage has built up. the control arrangement is fed from the tertiary winding L13 via the diode V10 and via the diode V6 and the resistor R13, the requisite auxiliary voltage being set up across the capacitor C6. Here the Zener diode V1 protects the operational amplifiers Dl, D2 and D3 from any excess voltage. WHAT WE CLAIM IS:

1. A D.C.-D.C. converter in which at least one periodically operating switching transistor is connected in series with a transformer primary winding that lies in the collector circuit of the switching transistor, and from a secondary winding of which at least one stabilised d.c. voltage is obtained by rectification, an outer regulating circuit being provided to control the base current to the switching transistor and effect D.C. output voltage stabilisation, and an inner regulating circuit being provided which acts each time the switching transistor has been rendered conductive to determine the disconnection time thereof, the collector-emitter voltage of the switching transistor being used as a controlling value, such that when it overshoots a given theoretical value disconnection of the base current to the switching transistor is effected, said theoretical value corresponding to a voltage at which the transistor is no longer overaturated. wherein the base of the switching transistor is preceded by a control switching amplifier stage consisting of a control amplifier transistor and an auxiliary control transformer. the primary winding of the auxiliary transformer being in series with the collector-emitter path of the control amplifier transistor. which is connected to the output of the outer regulating circuit. and the output signal from the inner regulating circuit being connected to the base of the control amplifier transistor, whilst a secondary winding of the auxiliary transformer is connected in the base circuit of the switching transistor.

2. A converter as claimed in Claim 1, in

which those ends of the two windings of the auxiliary transformer which are free of any high frequency potential are each poled in like manner, and that this auxiliary transformer possesses an airgap which is such that it is able to intermediately store the energy for the base operation of the switching transistor.

3. A converter as claimed in Claim 2, in which the inductance of the auxiliary transformer is selected to be sufficiently low to ensure that during each switching period of the control transistor a significant change in the current flowing through the secondary winding automatically occurs.

4. A converter as claimed in any preceding Claim, in which the primary winding of the auxiliary transformer which is connected in the collector circuit of the control transistor is connected in series with the emittercollector path of a further transistor whose base is operated from the output of an operational amplifier in dependence upon the D.C. output voltage.

5. A converter as claimed in any preceding claim, in which the primary winding of the auxiliary transformer is connected to said further transistor via a diode.

6. A converter as claimed in any preceding claim, in which a further diode is arranged in the emitter circuit of the control transistor.

7. A converter as claimed in any preceding claim, in which a series RC-element is arranged in parallel with the primary winding of the auxiliary transformer.

8. A converter as claimed in any preceding claim, in which a storage capacitor is arranged in parallel with the control switching amplifier stage.

9. A converter as claimed in any preceding claim, in which a resistor is connected in parallel with the secondary winding of the auxiliary transformer.

10. A converter as claimed in any preceding claim, in which a pulse generator is provided which operates on the starting of the converter to supply pulses to a monitoring value input of the inner regulating circuit.

II. A converter as claimed in Claim 10, in which that switching means are provided which block the pulse generator output pulses during normal operation of the switching transistor.

12. A converter as claimed in Claim 11, in which said switching means comprise a diode connected in parallel with said pulse generator.

13. A converter as claimed in any preceding claim, in which the outer and the inner regulating circuits are fed via an auxiliary secondary winding of said transformer whose primary winding lies in the collector circuit of the switching transistor.

14. A converter as claimed in any preceding claim, in which respective operational amplifiers provided in the outer regulating circuit, the inner regulating circuit, and the pulse generator are protected from excess voltage by a common parallel-connected Zener diode.

15. A D.C.-D.C. converter substantially as described with reference to Figures 1 and 2.